Crop Residue Decomposition and Nitrogen Mobilization

While there are several factors that affect the resulting amount of residue at planting, decomposition plays a role in both percentage of residue remaining as well as nitrogen (N) availability during and after decomposition. In corn, it is estimated that approximately 95% residue cover remains after harvest. When no other fall or winter management practices are factored in, winter decomposition alone has been found to drop the percentage coverage to roughly 86%.⁴

The process of crop residue decomposition is important because of its influence on the subsequent crop. There are three primary areas of risk that crop residue poses to the next crop: seedbed conditions, disease potential, and N immobilization. The presence of too much crop residue at planting puts the new crop at risk for soil conditions that are too cool and moist for optimal emergence and can harbor disease-causing pathogens. Additionally, if the bulk of residue decomposition is occurring during the growing season, N required for that process can limit N availability for crop growth and development. Understanding how crop residue breaks down and the factors that influence it can help farmers manage crops and crop residues to their benefit.

Crop residue is composed of lignin, cellulose, hemicellulose, and nutrients. In order for residue to decompose, many biological and chemical processes take place that are influenced by environmental and soil conditions such as air and soil temperature, soil moisture, pH, oxygen level, and soil microbial community. This nutrient cycling is a complex process that takes differing amounts of time based on the type of residue.

Residue decomposition includes the processes of N immobilization and mineralization, both of which involve soil microbes (Figure 1). During decomposition, soil microbes feed upon the carbon (C) in crop residue and require N for the process. Immobilization occurs when N is consumed by soil microbes and is not available to plants. Mineralization is a soil microbe mediated release of N from organic to inorganic sources. A higher concentration of C as compared to N will result in soil microbes taking a longer amount of time to break down the organic material and using more soil N to do their work.

Soil microbes prefer a C:N ratio of around 10:1, but the ratios of different crop residues vary greatly. Soybean, alfalfa, and other legumes generally have a C:N ratio near 15:1, thus resulting in faster mineralization. Crop residues with higher C:N ratios, such as corn, take more time to decay and result in higher amounts of N being required by the microbes from the soil solution to lower the C:N ratio.2 If not taken into account, the microbe requirement for N can compete with a growing corn crop for available N to maintain their desired C:N ratio of 10:1.3 Nitrogen deficiency symptoms can occur during this period of immobilization.

While cropping system and ecosystem management can influence the factors critical to the processes of residue decomposition, there is little that can be done to “manage” for optimal decomposition. It has long been thought that influencing the size of the residue or its amount of contact with the soil could hasten the decomposition process, but recent research has proven that is not the case.

A three-year study conducted in Iowa evaluated the effects of tillage on residue breakdown. The study showed no significant differences in decomposition or the percentage of residue remaining after 12 months among deep tillage, strip-tillage, and no-till systems.⁵ Another practice thought to hasten decomposition was to apply N fertilizer to residue after harvest seeking to stimulate soil microbes and speed the process. The Iowa research also evaluated the effects of both temperature and N application on the rate of residue decomposition. Observations from both field and laboratory studies showed that temperature had an effect on the rate of decomposition; a slower rate of decomposition was observed at low temperatures and it increased with higher temperatures. There was, however, no difference in residue decomposition with different rates of N added.5 These findings confirm that neither the use of tillage nor N application work to speed the decomposition process, and they may actually be counterproductive from an economic and environmental standpoint.

That being said, although tillage does not hasten the decomposition process, it does still serve a purpose in many cropping situations. One example is the use of strip-tillage to remove residue from the row for improved seedling emergence. Removal of debris from the strips of soil where seeds will be planted can help to warm soil faster and eliminate some of the physical barriers to seedling emergence (Figure 2.).

So, what can a farmer do to encourage residue breakdown in fields? Use of management practices that enhance soil health and microorganism populations can help encourage residue breakdown. Use of cover crops can provide additional energy, C, and N to the soil helping to sustain a wide range of soil microorganisms. It has also been proven that both time and temperature influence the rate of residue decomposition. While both of these factors are out of our control, one way to use time to your advantage though is to consider crop rotation. Rotation cycles that include legumes, such as soybean, can put N back into the soil more quickly and give corn residue more time to decompose.

Many biological and chemical processes take place during the course of crop residue decomposition. Soil microbes feed upon the C in crop residue and require N for the process. A higher concentration of C as compared to N will result in soil microbes taking a longer amount of time to break down the organic material and using more soil N to do their work. Recent research findings have shown that neither the use of tillage nor N application to residue contribute to increasing the rate of decomposition. However, the use of cover crops and crop rotation can help to build healthy soils and microorganism populations that encourage residue breakdown.